The present invention is a rubber additive comprising a low molecular weight polyethylene wax. The additive is used in rubber compounding and increases the processability of the rubber compounds and also enhances the rubber compound""s resistance to attack by ozone.
Rubber is inherently difficult to process and mold because of its high viscosity. Rubber is also subject to degradation and attack by ozone and ultraviolet (xe2x80x9cUVxe2x80x9d) light. Both of these phenomena are well known to those practiced in the art of compounding rubber.
Rubber formulations also contain a number of additives, which are used for purposes, such as, but not limited to, improve the processability of the rubber, and reduce the degradation of rubber caused by the effects of ozone and UV light.
Traditional additives used to improve processability of rubber have included polyethylene waxes. A commonly used polyethylene wax for use in improving process ability is commercially available from Honeywell (Morristown, N.J.) and is sold under the A-C(copyright)617 name. This polyethylene wax is a relatively soft with a hardness (@25 degrees C.) of 6-9 dmm, a Mettler drop point of 101 degrees C., a molecular weight of approximately 3000, and a density of 0.91 g/cc. There are several other suppliers of similar polyethylene waxes including Eastman Chemical (Kingsport, Tenn.) Epolene(copyright) wax and Mitsui (Tokyo Japan) Hi-Wax(trademark)
Polyethylene waxes are derived from polymerization of ethylene under conditions that restrict the polymer length there by rendering the final product with wax like characteristics. The traditional polyethylene waxes used in rubber processing have molecular weights in the 2000 to 3000 Mn range. Traditional polyethylene waxes used as processing aids in rubber compounds have been used sparingly because of their effects on reducing adhesion and green tack of rubber. In fact suppliers promote traditional polyethylene waxes as tack reducing compounds for those instances where reduced tack is desirable. In most instances, however, such as tire building, high levels of green tack aids in building of tires and is generally a desirable property.
To improve the resistance of rubber to the effects of ozone and UV light, rubber compounders have traditionally used combinations of static and dynamic antidegradants. Commonly used dynamic antiozonants include 2,4,6-Tris-(N-1,4-Dimethylpentyl-p-Phenylenediamino)-1,3,5-Triazine supplied by Uniroyal Chemical under the brand name Durazone(copyright)37, or N-isopropyl-Nxe2x80x2-phenyl-p-phenylenediamine- supplied by Monsanto Chemicals. Another dynamic antiozonant is N-(1-methylhexyl)-Nxe2x80x2-phenyl-p-phenylenediamine (6PPD), which is supplied by several chemical companies.
Rubber additives that are used to protect against static ozone attack are microcrystalline waxes that are sold under a variety of tradenames, such as Astor(copyright) (Honeywell, Inc., Morristown N.J.); Norcheck(copyright), by IGI International. Antiozonant microcrystalline waxes are derived from petroleum, and refined from slack wax to fractionate and separate out the microcrystalline fraction. The molecular weights of microcrystalline waxes are typically in the 600-700 Mn range. The microcrystalline waxes are used to provide a physical barrier on the surface of a rubber article, such as a tire. This physical barrier prevents attack on the rubber from ozone present in the atmosphere. The surface film of microcrystalline wax is sacrificial and is constantly being regenerated through a phenomenon called blooming. Blooming is the process where the microcrystalline wax, due to its incompatibility with rubber, continually migrates to the rubber surface. The rate of migration is a complex situation effected by time, heat, concentration of wax, chemical make-up of the rubber, and other factors. Microcrystalline wax use in tires has been limited due to the adverse effect high levels have on adhesion and green tack of the rubber compound.
Various applications of polyethylene waxes and microcrystalline waxes have been described in the prior art.
In U.S. Pat. No. 4,161,202 Powell et al. disclose using a low molecular weight (approximately 2000) polyethylene wax, having a melting point of approximately 78 degrees C., as an internal coating in a tire to render the tire puncture resistant. The polymer has the properties of a stiff grease which will not liquefy, but which will penetrate voids such as a puncture under the influence of inflation pressure.
Messerly et al. (U.S. Pat. No. 4,096,898) disclose using low molecular weight polyethylenes (molecular weights ranging from 1000-50,000, and having a density of approximately 0.88), as an internal tire lubricant. The compound is described as being a polyolefin grease, becoming liquid at a temperature of approximately 85 degrees C.
Ganster et al (U.S. Pat. No. 4,309.378) discloses using polyethylene wax as a release agent in tire manufacturing. The particular polyethylene is a polyethylene adipate having a molecular weight of approximately 2000.
In U.S. Pat. No. 3,992,502, Krishman discloses the use of polyethylene or polyethylene waxes as mold release agents in the formation of rubber products, to prevent adhesion of the finished product to the mold.
In U.S. Pat. No. 4,082,706 Danielson describes problems that occur when waxes are used as an antiozonant in a rubber formulation. He notes the limited success of waxes because of the difficulty of insuring that the wax layer remains intact. The wax film often separates or tears, causing cracks to develop of greater magnitude than articles having no wax, under the stresses to which the articles are subjected. The antiozonant compounds described are enamines.
Wheeler et al. (U.S. Pat. No. 4,956,405) describe the use of waxes to inhibit ozone cracking in articles under stress in static conditions by incorporating the wax into rubber compounds before vulcanization. They indicate the wax migrates to the surface of the article, forming a film which acts as a physical barrier to protect the article from ozone attack. The problem they note is that the wax film becomes cracked or disrupted during use of the article, and may cause cracks of greater severity than if no wax were used in the formulation, leading them to state xe2x80x9cfor many service conditions, the use of wax is impractical due to the dynamic conditions under which the article is expected to performxe2x80x9d(col. 1, lines 63-col. 2, line 7). The novel antiozonant compounds of this invention are tris-(N-alkyl-p-phenylenediamino)-1,3,5-triazines, and their use in a variety of elastomeric products, such as industrial belts, hoses, air springs, and roofing membranes are described.
In U.S. Pat. No. 5,120,779 Cornell et al. disclose the use of novel triazine compounds as antiozonants for rubber. Noteworthy is the statement at col. 2, lines 13-16 that the novel arylenediamine triazine of the invention provided exceptional long term ozone protection under static conditions without using wax. At col. 9, lines 5-8 Cornell et al. state that their inventive compounds may be used in combination with other antiozonant agents, and less preferably with microcrystalline waxes as are commonly used to protect against static ozone attack.
U.S. Pat. No. 6,201,049 Sakamoto et al.)discloses the use of N-(1-methylheptyl)-Nxe2x80x2-phenyl-p-phenylenediamine (xe2x80x9c6PPDxe2x80x9d) and N-(1,3-dimethylbutyl)-Nxe2x80x2-phenyl-p-phenylenediamine (xe2x80x9c8PPDxe2x80x9d) as antioxidant agents in a rubber composition used for tire sidewalls.
In U.S. Pat. No. 4,696,753 Umland et al. disclose the use of between 50-70 per cent by weight of polyethyleneglycol or polyglycol ether in combination with aluminum bronze, in conjunction with a wetting agent, as a lubricant for a tire and wheel assembly. The lubricant is applied as a thin layer, external to the tire, between the tire and rim assembly, to prevent damage or destruction of the tire due to slippage when the tire is operated for a long period of time in a deflated state.
In U.S. Pat. No. 4,501,616 Fink et al. disclose the use of polyoxypropylenediols or polyoxypropylenepolyoxyethylenediols, of molecular weights greater than 6000, as a lubricant/release agent which is added to the rubber before vulcanization during the tire manufacturing process.
In U.S. Pat. No. 4,666,518 Hallenbeck et al. disclose the use of a wax mixture for inside and outside tire paints for uncured rubber tires. The mixture contains different ratios of low melting temperature and high melting temperature waxes, such as paraffin and carnauba waxes, respectively. The tire paints contain release agents which enable the cured rubber tires to be removed from the tire mold.
Hartz (U.S. Pat. No. 4,137,358) discloses the use of a hydrocarbon wax as an additive to tire fabrics (such as tire cords) to protect them from adverse effects of exposure to atmospheric oxidation, as well as to adverse effects from combustion products present in fuel-oil or kerosene-fired dryers and curing ovens. The preferred wax has a molecular weight of approximately 1 500 and a melting point of point of between 160-215 degrees F. The waxes are comprised of mostly straight chain linear paraffins with molecular weights ranging from about 300 to about 1500.
Majumdar, in U.S. H1,871 discloses elements of the manufacturing of rubber tires. and indicates that addition of waxes as antiozonants can impair the tack of the curable rubber compounds when added in an amount that exceeds their solubility.
Therefore, there is a need for employing a low molecular weight polyethylene wax in rubber formulations that can improve the processability of the rubber formulation, and which can increase the resistance of that formulation to environmental factors, such as ozone.
The present invention is an additive for use in rubber tires. The product is a commercially available low molecular weight polyethylene wax, sold by Marcus Oil and Chemical under the tradename M300. Product M300 is a low molecular weight polyethylene homopolymer having an average molecular weight of approximately 1000-1100, and a melting point of 116 degrees C. Compound M300 was used in rubber compounds for automotive use, and found to increase the processability of the compound, and to enhance the rubber compounds"" resistance to attack by ozone.
It is an object of the present invention to provide an additive which can increase the processability of rubber compounds.
Another object of the present invention is to provide an additive which can increase the resistance of rubber goods to attack by atmospheric agents such as ozone.
Still another object of the present invention is to provide a rubber formulation which has increased processability.
Another object of the present invention is to provide a rubber compound which has increased resistance to attack by atmospheric agents such as ozone.
Another object of the present invention is to provide a low molecular weight polyethylene wax which can be economically produced.
The present inventors have unexpectedly discovered that a high density polyethylene with very low molecular weight (900-1,500 Mn) can be used effectively in rubber compounds to both reduce the degradation effects of ozone on the rubber article and provide for improved processing of the rubber compound without the negative effect of reducing green tack of the unvulcanized rubber compound.
The present invention relates to a rubber composition obtained by blending 1 to 3.5 phr (parts per hundred parts by weight of rubber) of a low molecular weight (Mn 900-1500), high density polyethylene wax while eliminating all of the traditional polyethylene wax (Mn 2000-3500) and substantially reducing the amount of microcrystalline wax used in the compound.
The resulting rubber compound has significantly improved tack verses prior art which aids in tire building as well as the fabrication of any built up rubber article (i.e. hose and belts). The compound also exhibits superior Static Ozone Resistance when compared to prior art. Additionally the favorable processing characteristic exhibited by the traditional polyethylene processing aids is maintained through the use of the present invention.